Rewritable optical discs, such as CD-RW, DVD+R, DVD-R, DVD-RAM, and BD-R contain a
recording layer that consists of a stack of thin films. At the center of
this stack is a phase change material, such as the quaternary alloy AgInSbTe.
The unrecorded or erased state of the material is crystalline. The
recorded data marks are amorphous. It is possible to image the data marks
by removing the overlying films so that the metal layer is exposed. When
this is done, the AFM can distinguish the crystalline and amorphous regions by a
small difference in surface electric potential.

Surface Potential (Kelvin probe) image of CD-RW disc after mechanical
delamination to expose the metallic film. The surface potential was
about 50 mV higher in the amorphous data marks (bright yellow) than in the
surrounding crystalline material.